1. Technical Field
The present invention relates generally to an improved method of and system for profile equalization employing visible laser diode (VLD) displacement during image formation and detection operations, and also to an improved method of and system for producing digital images using such improved object illumination.
2. Brief Description of the Prior Art
The use of image-based bar code symbol readers and scanners is well known in the field of auto-identification. Examples of image-based bar code symbol reading/scanning systems include hand-hand scanners, point-of-sale (POS) scanners, and industrial-type conveyor scanning systems.
Presently, most commercial image-based bar code symbol readers are constructed using charge-coupled device (CCD) image sensing/detecting technology. Unlike laser-based scanning technology, CCD imaging technology has particular illumination requirements which differ from application to application.
Most prior art CCD-based image scanners, employed in conveyor-type package identification systems, require high-pressure sodium, metal halide or halogen lamps and large, heavy and expensive parabolic or elliptical reflectors to produce sufficient light intensities to illuminate the large depth of fields supported by such industrial scanning systems. Even when the light from such lamps is collimated or focused using such reflectors, light strikes the target object other than where the imaging optics of the CCD-based camera are viewing. Since only a small fraction of the lamps output power is used to illuminate the CCD camera's field of view, the total output power of the lamps must be very high to obtain the illumination levels required along the field of view of the CCD camera. The balance of the output illumination power is simply wasted in the form of heat.
Most prior art CCD-based hand-held image scanners use an array of light emitting diodes (LEDs) to flood the field of view of the imaging optics in such scanning systems. A large percentage of the output illumination from these LED sources is dispersed to regions other than the field of view of the scanning system. Consequently, only a small percentage of the illumination is actually collected by the imaging optics of the system. Examples of prior art CCD hand-held image scanners employing LED illumination arrangements are disclosed in U.S. Pat. Nos. 5,777,314, 5,756,981, 5,627,358, 5,484,994, 5,786,582, and 6,123,261 to Roustaei, each assigned to Symbol Technologies, Inc. and incorporated herein by reference in its entirety. In such prior art CCD-based hand-held image scanners, an array of LEDs are mounted in a scanning head in front of a CCD-based image sensor that is provided with a cylindrical lens assembly. The LEDs are arranged at an angular orientation relative to a central axis passing through the scanning head so that a fan of light is emitted through the light transmission aperture thereof that expands with increasing distance away from the LEDs. The intended purpose of this LED illumination arrangement is to increase the “angular distance” and “depth of field” of CCD-based bar code symbol readers. However, even with such improvements in LED illumination techniques, the working distance of such hand-held CCD scanners can only be extended by using more LEDs within the scanning head of such scanners to produce greater illumination output there from, thereby increasing the cost size and weight of such scanning devices.
Similarly, prior art “hold-under” and “hands-free presentation” type CCD-based image scanners suffer from shortcomings and drawbacks similar to those associated with prior art CCD-based hand-held image scanners.
Recently, there have been some technological advances made relating to planar laser illumination techniques employed in vision-based industrial bar code scanners, in particular, U.S. Pat. Nos. 6,629,641, 6,631,842, 6,732,929, 6,736,321, 6,742,711, 6,764,008, 6,786,414, and 6,837,437 being commonly owned by Assignee, Metrologic Instruments, Inc., of Blackwood, N.J., and incorporated herein by reference in it's entirely.
There have also been some technological advances made relating to the laterally aiming of laser illumination techniques in CCD-based image capture systems, employed to flatten illumination profiles across the width of conveyor belts, in particular, U.S. Pat. No. 6,732,929. A consideration when designing laser illumination techniques in CCD-based image capture systems is the equalization of the illumination profile from near field to far field, such that the image brightness is substantially the same independent of object distance from the image scanner. As disclosed in U.S. Pat. No. 6,732,929, this can be accomplished, at least in part, by controlling the width of the laser line, in particular by causing the laser line to decrease in width from near field to far field. In this manner, the gain in power density due to the decreasing laser line width offsets the loss in power density due to the increasing laser line length, resulting in equalized profiles throughout the depth of field.
However, there may be situations in which controlling the laser line width is insufficient to achieve complete equalization of the illumination profiles. Depending on the constraints of the application, it may not be possible to achieve the desired laser line widths at the necessary distances from the image scanner. Thus, there is a great need in the art for an improved method of and system for profile equalization.